This the U.S. National phase of international application PCT/SEO2/01238 filed in English on 20 Jun. 2002 which designates the U.S. PCT/SE02/01238 claims priority of SE Application No. 0102345-6 filed 29 Jun. 2001. The entire contents of these applications are incorporated herein by reference.
The present invention pertains to the field of feed forward amplifying circuits, and particularly to feed forward amplifying circuits having signal delay compensation.
A feed forward amplifying circuit is one form of amplifying circuit which employs linearisation of an amplifier which operates in a non-linear mode. The feed forward amplifying circuit is used in many technical applications, such as radio applications (e.g. high frequency military communication equipment, cellular radio communication systems satellite systems etc) or cable TV applications. The basic design of the feed forward amplifying circuit is fairy simple (first described in 1927 by H. S. Black of Bell Telephone Laboratories). The feed forward amplifying circuit includes a first branch with a main amplifier operating in a non-linear (near-linear) mode. A second (feed forward) branch is connected to the first branch in order to linearise the operation of the circuit. The linearisation takes place in two steps. In a first step, an error signal, which is indicative of distortion generated by the main amplifier, is generated on the basis of an input signal to the feed forward amplifying circuit and an output signal from the main amplifier. In a second step, the error signal is amplified by (linear) amplifying means arranged in the second branch and combined with the output signal from the main amplifier in a manner which eliminates the distortion from said output signal. Consequently, only a linear amplification of the input signal remains.
The electronics (e.g. the main amplifier) in the first branch as well as the electronics (e.g. the linear amplifying means) in the second branch introduce some signal delay in the branches.
These signal delays will influence the operation of circuit negativelyxe2x80x94the wider the bandwidth of the input signal, the more troublesome the influence of the signal delays. It is, therefore, common for the feed forward amplifying circuit to include signal delay compensation means for providing time alignment between the two branches. The signal delay compensation means normally includes fixed delay elements, e.g. fixed delay lines, such as transmission lines, of appropriate lengths. The fixed delay elements compensate for most of the signal delays introduced by the electronics, but usually there are some remaining signal delay differences between the two branches which must-also be compensated for, especially in wide-bandwidth operation. This additional xe2x80x9cfine tuningxe2x80x9d of the signal delay compensation is traditionally performed by a testing procedure where different lengths of coax interconnection cables are tried in the feed forward amplifying circuit until an acceptable signal delay compensation is achieved. There are, however, some disadvantages with this approach. First there is the logistic issue, since a large number of different cable assemblies are necessary in order to cover delay spread. Furthermore, the coax cables have to be changed manually when the signal delay compensation of the feed forward amplifying circuit is tuned. This of course incurs increased costs. The testing is also interrupted when the coax cable is changed leading to increased test time. Consequently, the described approach is not fully suitable for industrial production of feed forward amplifying circuits.
The present invention addresses mainly a problem of obtaining a feed forward amplifying circuit with signal delay compensation, wherein tuning of the signal delay compensation can be performed easily, quickly and cheaply, thus making the feed forward amplifying circuit more suitable for industrial production.
The above-stated problem is solved in short with a feed forward amplifying circuit according to the following. The feed forward amplifying circuit comprises signal delay compensation means including at least one variable signal delay element. The at least one variable signal delay element includes a plurality of signal delay components and switching circuitry providing a possibility of introducing at least one signal delay components, selected from the plurality of signal delay components, into a signal delay path.
A main advantage with the invention is that tuning of the signal delay compensation becomes easy. The tuning is performed with the at least variable signal delay element. Consequently, there is no need for the costly and time consuming procedure including coax cable xe2x80x9cswappingxe2x80x9d as described above. The feed forward amplifying circuit according to invention is therefore more suited for production on an industrial scale.
The invention will now be described further using preferred embodiments and referring to the drawings.